The conventional NTSC color television system used in the United States is notoriously well known. This system was established at a time when the implementation of electronic functions required vacuum tubes, and complex electronic functions could not be implemented in commercial equipment because of cost and reliability considerations. Thus, the NTSC standards were selected in consideration, among other considerations, of simplicity, and of accommodation of the idiosyncrasies of the electronic components available at that time.
Due to the deficiencies of early color cameras, transmitters and receivers, color hues were often incorrectly reproduced, and other image distortions were common. When European nations adopted color television standards, they attempted to alleviate some of the perceived problems by adopting a phase alternate line scheme (PAL), and by making other modifications such as increasing the number of displayed lines of video and/or accommodating different power-line frequencies by amending the timing standard. As a result of these differences, color television receivers for the various well-known television systems are incompatible with transmitters broadcasting other standards.
In the time since inception of the current television standards, electronic components have been significantly improved. For example, reliable transistors supplanted vacuum tubes, and integrated circuits have been developed that allow inexpensive receivers to perform complex processing functions.
Television cameras have also been improved, with various reliable, sensitive, low-voltage solid-state imagers such as charge-coupled device (CCD) imagers supplanting the earlier vacuum-tube imagers, with their high voltage requirements, magnetic windings, and alignment problems. The text Charge Transfer Devices, by Sequin et al., published by Academic Press, 1975, describes the general characteristics of charge transfer devices and charge coupled device (CCD) imagers. The CCD imagers have an image sensing portion including an array of light sensitive pixels arrayed in rows and columns for building up charge during an integrating interval, an image storage portion into which the charge from the sensor portion is rapidly transferred during the vertical blanking (VB) interval, and a horizontal transfer portion which reads the image from the storage portion line by line.
While the vacuum-tube imagers in cameras are being supplanted by solid-state devices, the image display device in a conventional receiver continues, for the most part, to be a vacuum-tube device, namely the kinescope, which, much like the earlier cameras, is bulky and fragile, requires high operating voltages, includes magnetic windings, and requires alignment. The timing of the original television standards is still relevant to the requirements of the kinescope. Some solid-state image display devices are known, but they have not become commercially widespread either because of their high cost or because of various deficiencies such as lag and poor contrast.
As a result of the perceived limitations of the current television standards, attention has been directed to various high-definition television (HDTV) systems, with the hope of supplementing or supplanting the current system with a new television system having enhanced capabilities and performance. Many new systems have been devised and proposed, including a proposed new digital HDTV system for the United States, with 1920 active picture elements (pixels) per line, and with 1080 active horizontal lines per frame. This system, as with other recently proposed systems, includes so may improvements over the current NTSC system, including a wider aspect ratio, that its timing and format are incompatible with NTSC.
New solid-state cameras are being developed to generate signals of a quality commensurate with the new HDTV systems. The number of horizontal scan lines of the image is being increased from the 525 lines required by NTSC, the number of pixels in each horizontal scan line (which corresponds to the number of columns of pixels in the imager) is also increased to provide images with enhanced horizontal resolution, and other deficiencies or problems, such as smear, are being addressed. As the number of light sensor pixels in a camera imager becomes greater, the sensor pixels themselves become smaller (unless the imager size is increased), so the light which each pixel receives is reduced, and the signal-to-noise ratio (SNR) therefore tends to degrade. Increasing the number of scan lines and the number of pixels in the camera imager requires increasing the pixel read rate so that the signals representing a complete image become available at the camera output within the allotted time, and the clock period must therefore be reduced (the clock frequency must be increased), by comparison with an imager for a current standard.
The problem of signal-to-noise ratio of the video signal has been addressed in many ways, one of which is by the use of the "Hole Accumulated Diode" (HAD) structure, which tends to reduce the noise signal, and thereby restore the SNR notwithstanding the smaller sensors. The solution of increasing the read clock rate in order to read the increased number of pixels in the allotted time, however, has not been totally successful, in part because the number of pixels which must be addressed in the imager is so greatly increased.
Early CCD imagers had about 260 pixels per line, and 262 horizontal lines, for a total of about 70,000 pixels, whereas the newer requirements are on the order of 1125 horizontal lines, and 1920 pixels per line, for a total of about 2,200,000 pixels, which is more than twenty times the earlier number. Consequently, clock frequencies which are selected to transfer the charge from all the pixels of the camera imager to the storage region within the allowed time tend to be high enough to be near the limits of the speed at which charge can be transferred within the CCD imager itself. Operation at these transfer clock frequencies, therefore, tends to introduce distortion, which may in part result from incomplete charge transfer during each transfer clock cycle.
The desired number of pixels for HDTV images may be achieved at lower clock rates by using a plurality of CCD imagers with overlapped pixels viewing the same scene, but this imposes additional costs in that two imagers must be used to generate the signal for one color, they must be registered together, and additional control and signal combining circuits must be provided.